file-handling

File operations

What are file-processing operations?

• File-processing involves interacting with external files stored on secondary storage (e.g. hard drives, SSDs)

• In programming, files must be properly opened, read/written, and closed to ensure data is handled correctly

• Different types of file access are also used

Common file operations

Types of file access

Programming file-operations

Pseudocode

Basic file operations

OPEN filename FOR mode // mode: READ, WRITE, APPEND
READ record FROM filename // Read one record
WRITE record TO filename // Write one record
CLOSE filename // Always close the file

Serial access (e.g. logging events)

• Data is written one after another (no specific order)

• Often used with APPEND mode

Example: Writing logs serially

TYPE LogRecord DECLARE message : STRING DECLARE timestamp : STRING
ENDTYPE DECLARE log : LogRecord SET log.message = "System started"
SET log.timestamp = "12:00" OPEN "logfile.txt" FOR APPEND
WRITE log TO "logfile.txt"
CLOSE "logfile.txt"

Sequential access (e.g. reading through all records)

• Records are read in order from start to end

• Common for reports or summaries

Example: Reading student grades sequentially

TYPE Student DECLARE name : STRING DECLARE grade : INTEGER
ENDTYPE DECLARE student : Student OPEN "students.txt" FOR READ WHILE NOT EOF("students.txt") READ student FROM "students.txt" OUTPUT student.name, student.grade
ENDWHILE CLOSE "students.txt"

Random access (e.g. accessing specific records by key)

• Accesses records directly using a key (e.g. record number or unique ID)

• Requires indexed files or fixed-length records

Example: Update a student’s grade by record position

TYPE Student DECLARE name : STRING DECLARE grade : INTEGER
ENDTYPE DECLARE student : Student
DECLARE recordNumber : INTEGER SET recordNumber = 5 // go directly to the 5th record OPEN "students.txt" FOR RANDOM
SEEK "students.txt", recordNumber
READ student FROM "students.txt" SET student.grade = 90 // update grade SEEK "students.txt", recordNumber
WRITE student TO "students.txt"
CLOSE "students.txt"

Summary

Python

Basic file operations

file = open("filename.txt", "r" or "w" or "a") # Open for read, write, or append
line = file.readline() # Read one line
file.write(data) # Write one line
file.close() # Close the file

Serial access (e.g. logging events)

Example: Appending log entries to a file (serial write)

from datetime import datetime message = "System started"
timestamp = datetime.now().strftime("%H:%M") log = f"{timestamp} - {message}n" with open("logfile.txt", "a") as file: file.write(log)

Sequential access (e.g. reading through all records)

Example: Reading through each student line by line

with open("students.txt", "r") as file: for line in file: name, grade = line.strip().split(",") print(f"Name: {name}, Grade: {grade}")

Random access (e.g. accessing specific records by key)

Example: Update a student’s grade by index (record number)

record_number = 4 # 5th record (0-indexed) with open("students.txt", "r") as file: records = file.readlines() name, grade = records[record_number].strip().split(",")
records[record_number] = f"{name},90n" # Update grade to 90 with open("students.txt", "w") as file: file.writelines(records)

Summary

Java

Basic file operations

BufferedReader reader = new BufferedReader(new FileReader("filename.txt"));
String line = reader.readLine();
reader.close(); BufferedWriter writer = new BufferedWriter(new FileWriter("filename.txt", true)); // true = append
writer.write("Some text");
writer.newLine();
writer.close();

Serial access (e.g. logging events)

Example: Appending log entries to a file (serial write)

import java.io.*;
import java.time.LocalTime;
import java.time.format.DateTimeFormatter; public class SerialAccessExample { public static void main(String[] args) throws IOException { String message = "System started"; String timestamp = LocalTime.now().format(DateTimeFormatter.ofPattern("HH:mm")); String log = timestamp + " - " + message; BufferedWriter writer = new BufferedWriter(new FileWriter("logfile.txt", true)); writer.write(log); writer.newLine(); writer.close(); }
}

Sequential access (e.g. reading through all records)

Example: Reading through each student line by line

import java.io.*; public class SequentialAccessExample { public static void main(String[] args) throws IOException { BufferedReader reader = new BufferedReader(new FileReader("students.txt")); String line; while ((line = reader.readLine()) != null) { String[] parts = line.split(","); String name = parts[0]; String grade = parts[1]; System.out.println("Name: " + name + ", Grade: " + grade); } reader.close(); }
}

Random access (e.g. accessing specific records by key)

Example: Update a student’s grade by index (record number)

import java.io.*;
import java.util.*; public class RandomAccessExample { public static void main(String[] args) throws IOException { List<String> records = new ArrayList<>(); int recordNumber = 4; // 5th line (0-indexed) BufferedReader reader = new BufferedReader(new FileReader("students.txt")); String line; while ((line = reader.readLine()) != null) { records.add(line); } reader.close(); // Update the grade String[] parts = records.get(recordNumber).split(","); String updatedRecord = parts[0] + ",90"; records.set(recordNumber, updatedRecord); // Overwrite the file BufferedWriter writer = new BufferedWriter(new FileWriter("students.txt")); for (String record : records) { writer.write(record); writer.newLine(); } writer.close(); }
}

Summary